Resistive memories (RRAM), such as oxide-based direct-access memories (OxRAM), electrolytic memories (CBRAM), or even ferromagnetic memories (FRAM), have many advantages and attributes. These include very short read and write times, low operating voltages, low power consumption, easy integration, an almost infinite endurance and a potentially very high density.
RRAM resistive memories typically include memory points capable of storing one bit, which points are distributed in rows and columns in a matrix array in a memory plane. A memory point is accessed via word lines traversing the rows of the memory plane and bit lines traversing the columns of the memory plane.
In RRAM resistive memories, each memory point usually includes a capacitive metal-oxide-metal (MOM) structure or memory cell. The metal layers of the capacitive MOM structure form top and bottom electrodes, which are placed on either side of a dielectric layer and may be metal oxide, for example.
Since they include oxide and metals, the capacitive memory cells may advantageously be fabricated in the interconnect portion of an integrated circuit. This portion is located above the substrate and usually designated in the art by the acronym BEOL (back-end of line).
The processes for fabricating a RRAM resistive memory utilize conventional photolithography steps, in which a layer of photoresist deposited on the structure being formed is irradiated in a desired pattern. Next, the resist that was irradiated (or that was not radiated) is removed to form a resist mask to etch the exposed portion of the structure through the resist mask.
FIG. 1 illustrates a conventional resist mask deposited on a resistive-memory memory plane PM during the manufacturing process, as seen from above. The resist mask includes “pads” 11 of square shape, periodically repeated in the directions of the rows X and the columns Y of the memory plane PM. The pads 11 eventually define the dimensions of the capacitive cells CEL.
In the photolithography steps, the projected images appear with irregularities, such as rounded corners. Furthermore, despite the implementation of optical proximity corrections (OPCs), the square portions of the resist have a tendency to round at their corners and to end up becoming circular, as shown by the dotted lines 12. Therefore, the area on which the resist pads rest decreases.
The smaller the area, the more the adhesion of the resist becomes problematic, and the greater the risk that debonding will generate severe fabrication defects. Thus, the densification of RRAM type memories may be limited by this problem of adhesion of the photoresist.